Conventionally, in a wheel loader of a example of the working vehicle, a vehicle main body is provided with a boom attached to the vehicle main body so as to move up and down, a working member such as a bucket or the like rotatably attached to the boom, and a working apparatus comprises a boom cylinder and a bucket cylinder respectively actuating the boom and the working member. Sediment is excavated, conveyed and loaded by actuating the boom and the working member.
In the wheel loader, in order to suppress a pressure pulsation and reduce vibration of a vehicle body for improving a ride quality and preventing a load collapse in the working apparatus at a time of traveling, for example, there is proposed a dynamic damper of a working vehicle shown in a patent document 1. As shown in FIG. 14, the dynamic damper of the working vehicle is structured as follows.
An elevating cylinder (hereinafter, referred to as a boom cylinder 112) is expanded and contracted by receiving pressure oil of a hydraulic pump 117 by a control valve 119, thereby moving the boom up and down. The control valve 119 is connected to a head side oil chamber 126 of the boom cylinder 112 via a piping 127, and is connected to a bottom side oil chamber 128 via a piping 129.
The piping 127 and the piping 129 connect a branch piping 130 and a branch piping 131 which are respectively branched in a middle of a pipe passages. The branch piping 130 is connected to an oil tank 116 via a switching valve 133. The branch piping 131 is connected to an accumulator 162 via a switching valve 133 and a variable throttle apparatus 161. The switching valve 133 is structured by an electromagnetic valve, urged by a spring when it is excited so as to be switched to a cutoff position, and switched to a connected position when it is not excited.
The variable throttle apparatus 161 is constituted by a throttle apparatus which can regulate a throttle opening degree in a plurality of stages, and comprises a plurality of throttles 164 and 165 and a throttle selecting switching valve 166. The switching valve 166 is constituted by an electromagnetic valve, and urged by the spring when it is not excited so as to be switched to a position selecting the throttle 164 having a large throttle opening degree. The switching valve 133 and the switching valve 166 are controlled by a controller 153.
A pressure sensor 149 connected to the piping 129 detects a pressure in a bottom side oil chamber 128. The controller 153 excites a coil in the switching valve 133 so as to switch to the connected position, at a time when the pressure in the bottom side oil chamber 128 detected by the pressure sensor 149 is within a range which is equal to or more than a minimum allowable pressure of the accumulator and equal to or less than a maximum allowable pressure. Further, the controller 153 selects a position of the switching valve 166 depending on the pressure detected by the pressure sensor 149, at a time when the pressure detected by the pressure sensor 149 is equal to or more than a set pressure.
The set pressure is set as follows. A proper set mass between a minimum mass of the installed working apparatus and a maximum mass of the working apparatus including a loaded material is assumed. For example, a mass which is one half of a sum of the minimum mass and the maximum mass is set to the set mass. On an assumption that the mass of the working apparatus is the assumed fixed mass, when the control valve 119 is at a neutral position, the pressure in the bottom side oil chamber 128 of the boom cylinder 112 at this time is defined as the set pressure.
In the structure mentioned above, the dynamic damper is actuated by turning on a changing switch 155 as well as setting the control valve 119 at the neutral position. Accordingly, it is possible to prevent a vibration such as a pitching, a bouncing or the like of the vehicle main body at a time of traveling of the wheel loader.
The wheel loader is traveled in a state in which the dynamic damper is actuated. At this time, the working apparatus is vibrated in response to a condition of a road surface, an acceleration and deceleration of the vehicle or the like, and the boom is going to oscillate in a vertical direction in correspondence thereto. Accordingly, a pressure fluctuation is generated within the oil chamber 128 in a bottom side of the boom cylinder 112 holding the boom.
The pressure within the bottom side oil chamber 128 at this time is detected by a pressure sensor 149. When the detected pressure is within a range equal to or more than the minimum allowable pressure of the accumulator 162 and equal to or less than the maximum allowable pressure, the controller 153 changes the switching valve 133 to the connected position.
In the case that the mass of the working apparatus is smaller than the set mass mentioned above, and the pressure within the bottom side oil chamber 128 is lower than the set pressure, the bottom side oil chamber 128 and the accumulator 162 are connected via the throttle 164 having a large throttle opening degree, due to the change of the switching valve 166. In a case that the mass of the working apparatus is equal to or larger than the set mass, and the pressure within the bottom side oil chamber 128 is equal to or higher than the set pressure, the bottom side oil chamber 128 and the accumulator 162 are connected via the throttle 165 having a small throttle opening degree due to the change of the switching valve 166.
Accordingly, even if the mass of the working apparatus is changed and a vibration characteristic is changed, it is possible to effectively suppress the vibration such as the pitching, the bouncing or the like of the vehicle main body by an actuation of the dynamic damper.
Patent Document 1: Japanese application laid-open publication No. 2001-200804